The present invention relates to neurostimulation systems (for example, a Spinal Cord Stimulation (SCS) system), and more particularly to a system and method for increasing the therapeutic ratio and usage range in a neurostimulator.
A spinal cord stimulation system treats chronic pain by providing electrical stimulation pulses through the electrodes of an electrode array placed epidurally near a patient's spinal cord. The therapeutic ratio is defined as the ratio of the maximum comfortable stimulation amplitude (numerator) to the just-perceivable stimulation amplitude (denominator). The therapeutic ratio is important because it is a measure of the range of adjustment allowed to a patient to meet varied pain relief/paresthesia requirements during the patient's daily activities. A large therapeutic ratio provides tolerance for changes in a patient's activities of daily living (posture variations, etc.) that may affect stimulation effectiveness. A low therapeutic ratio does not provide much tolerance for changes in the patient's activities.
Spinal cord stimulation is a well accepted clinical method for reducing pain in certain populations of patients. SCS systems typically include an Implantable Pulse Generator (IPG), electrodes positioned on an electrode lead, and (in some instances) an electrode lead extension. The IPG generates electrical pulses that are delivered to the dorsal column fibers and dorsal root fibers in the spinal cord, through the electrodes. The electrodes are implanted along the dura of the spinal cord. Individual electrode contacts (the “electrodes”) may be arranged in a desired pattern and spacing in order to create an electrode array. Individual wires connect with each electrode in the array. These wires are encased within an insulating material, and thus incased are generally referred to as a lead. Typically, the lead exits the spinal column and attaches to an electrode lead extension. The electrode lead extension, in turn, is typically tunneled around the torso of the patient to a subcutaneous pocket where the IPG is implanted. (In some instances, the lead that exits the spinal column may be connected directly to the IPG.)
SCS and other stimulation systems are known in the art. For example, an implantable electronic stimulator is disclosed in U.S. Pat. No. 3,646,940, that provides timed sequenced electrical impulses to a plurality of electrodes. As another example, U.S. Pat. No. 3,724,467 teaches an electrode implant for neuro-stimulation of the spinal cord. A relatively thin and flexible strip of biocompatible material is provided as a carrier on which a plurality of electrodes are formed. The electrodes are connected by a conductor, e.g., a lead body, to an RF receiver, which is also implanted, and which is controlled by an external controller.
In U.S. Pat. No. 3,822,708, another type of electrical spinal cord stimulation device is taught. The device disclosed in the '708 patent has five aligned electrodes which are positioned longitudinally on the spinal cord. Current pulses applied to the electrodes block sensed intractable pain, while allowing passage of other sensations. The stimulation pulses applied to the electrodes have a repetition rate of from 5 to 200 pulses per second. A patient operated switch allows the patient to change which electrodes are activated, i.e., which electrodes receive the stimulation pulses, so that the region between the activated electrodes on the spinal cord can be adjusted, as required, to better block the pain.
When an SCS system is implanted, a fitting procedure is performed to optimize the stimulation parameters for the particular patient. The stimulation parameters are optimized for both treatment efficacy, and to minimize power consumption. Disadvantageously, following optimization, the stimulation pulse width is generally fixed at some nominal value, and the patient is limited to control of the stimulation amplitude. Typically, such limited control has been shown to result in a therapeutic ratio of only about 1.4. As a result of such small therapeutic ratio, the patient must be careful to not adjust the stimulation level too abruptly or inadvertent over-stimulation may result. Also, if the patient's pain increases, such small therapeutic ratio may result in the patient exceeding the maximum comfortable amplitude to obtain pain relief.
What is needed is a method to increase the therapeutic ratio in order to provide more effective pain relief.